1. Field of the Invention
The present invention relates to a voice recording/reproducing device implemented by using an adaptive differential pulse code modulation method.
2. Description of the Related Art
The adaptive differential pulse code modulation (hereafter referred to as an xe2x80x9cADPCMxe2x80x9d) is widely used in digital modulation of voices. In the ADPCM method, sampled analog voice data is first converted to digital voice data, then a difference in the sampled data value between a digital voice data and a subsequent or neighboring digital voice data is quantized in accordance with the step for quantizing, i.e., a width for quantizing which is adaptively changed depending on an amplitude of a waveform of the voice signal to be encoded and 3 to 4 bits are used to express one quantized difference for encoding of the voice signal, thereby improving the followability to its original analog waveform.
FIG. 2 is a diagram explaining the ADPCM method in which 4 bits are used for quantizing and encoding of voice signals. An encoded digital voice data obtained by the ADPCM is formed by a multiple of polarity bits and quantizing width xcex94. The quantizing width xcex94 is decided by the correlation between the present sampled value and the past sampled value of the voice signals. In FIG. 2, the data provided by high order one bit represents a polarity (i.e., an increase or decrease of signals) and the data provided by low order three bits represents the multiple of the quantizing width xcex94. The data obtained by the ADPCM is stored in memory.
The digital voice data is reproduced by decoding the data obtained by the ADPCM. The digital voice data being a digital amplitude vale is calculated by the following formula (1):
An+1=Anxc2x1(Dxc3x97P)xe2x80x83xe2x80x83(1)
where, xe2x80x9cAn+1xe2x80x9d represents the digital amplitude value to be reproduced this time, xe2x80x9cAnxe2x80x9d represents a digital amplitude value reproduced at a previous time, xe2x80x9cDxe2x80x9d represents the multiple expressed by the low order bits of encoded data and xe2x80x9cPxe2x80x9d represents a parameter used to decide the quantizing width xcex94. The value (Dxc3x97P) shown as a product in the formula (1) represents the difference in the sampled data value between a digital voice data and a subsequent or neighboring digital voice data.
The digital voice data is converted to its analog voice data and is then outputted.
Since the ADPCM method has an advantage in that it can provide a good quality and it enables easy creation of voice data considering simple configurations required for implementing the ADPCM, it is used, for example, in IC recorders, telephones or the like. Moreover, the large reduction of the number of bits required for quantizing is made possible.
In the voice recording/reproducing device, for fast-forward reproduction, encoded data is split into voice blocks each having a predetermined size and stored at a time of recording. Then, at the time of the fast-forward reproduction, a part of voice blocks to be reproduced is removed or xe2x80x9cculledxe2x80x9d from the split voice blocks and encoded data of the voice block is decoded. When any part of the voice block is removed or culled from continued voice blocks for the fast-forward reproduction, the encoded data becomes discontinuous among voice blocks to be reproduced.
FIG. 3 is a diagram explaining operations of a voice recording/reproducing device implemented by the conventional ADPCM method. As shown in FIG. 3, if the voice blocks B and D are partially removed or culled from the whole voice blocks A, B, C, D and E in order to carry out the fast-forward reproduction, the discontinuity occurs between the blocks A and C and between C and E as a result.
In the voice recording/reproducing device implemented by the conventional ADPCM method, the digital amplitude value xe2x80x9cAn+1xe2x80x9d being a presently reproduced value is calculated by, for convenience""s sake, setting both a digital amplitude value xe2x80x9cAnxe2x80x9d being a just previously reproduced value and the parameter xe2x80x9cPxe2x80x9d to O. Because of this, at the time of the fast-forward reproduction, though, in the voice block B, its original voice signal is faithfully reproduced, in the voice block C, its original voice signal C1 is not reproduced faithfully and a reproduced voice signal C2 appears instead, i.e., at a head portion of the voice block C, the original voice signal cannot be faithfully reproduced, thus causing an inconvenience in that the reproduced voice cannot be clearly heard. The same thing occurs in the voice block E.
In view of the above, it is an object of the present invention to provide a voice recording/reproducing device capable of reproducing faithfully an original voice even at a time of performing fast-forward reproduction.
According to one aspect of the present invention, there is provided a voice recording/reproducing device implemented by using an adaptive differential pulse code modulation method including:
an analog-digital converter to sample analog voice signals and to convert them to digital voice data;
recording means for quantizing a difference in the sampled data value between a digital voice data and a subsequent and neighboring digital voice data obtained by the analog-digital converter in accordance with the width for quantizing corresponding to the difference, for obtaining encoded data by encoding the quantized difference and then for splitting the encoded voice data into voice blocks each having a predetermined size;
reproducing means for reproducing digital voice data, at a time of a fast-forward reproduction, by sequentially decoding specified data composed of the encoded data contained in the voice block to be reproduced and obtained by removing and culling a part of voice blocks from a series of the split voice blocks;
a digital-analog converter to convert reproduced digital voice data to analog voice signals, and
whereby the recording means is operated to add just previously sampled data and a parameter used to decide the width for quantizing to a head of the encoded data of each voice block and the reproducing means is operated, at the time of the fast-forward reproduction, to decode the head coded data of the voice block to be reproduced based on the additional data.
In the foregoing, a preferable mode is one wherein the data constituting the difference contains data of a multiple of the width for quantizing and of polarity causing an increase or decrease of the difference.
Also, a preferable mode is one wherein the encoded data contains data of the multiple of the width for quantizing and wherein the reproducing means is operated to decode the head digital voice data of each voice block by using data of the multiple of the width for quantizing contained in the encoded data, just previously sampled data and the parameter used to decide the width for quantizing.
Also, a preferable mode is one wherein the recording means is provided with an ADPCM (Adaptive Differential Pulse Code Modulation) analyzer.
Also, a preferable mode is one wherein the reproducing means is provided with an ADPCM synthesizer.
Also, a preferable mode is one wherein a format of the encoded data is composed of a voice start address region, a voice end address region and an ADPCM data region to its head of which the additional data are added.
Furthermore, a preferable mode is one wherein the format of the encoded data further includes a sampling frequency region.